Vegetable and animal fats and oils often contain considerable proportions of free fatty acids. The content of free fatty acids may be between 0 and 100% according to source of the fatty raw material. The proportion of free fatty acids cannot be reacted with methanol to give the corresponding fatty acid methyl esters in the preparation processes for biodiesel by transesterification of triglycerides with methanol and leads to yield losses or to the result that raw materials with a high content of free fatty acids are unsuitable for biodiesel production. A pretreatment of the fats is therefore necessary, in which the content of free fatty acids is reduced by conversion to fatty acid alkyl esters.
The literature discloses the esterification of the free fatty acids in fats or oils with methanol with the aid of a homogeneous acidic catalyst, for example p-toluenesulphonic acid. However, this process entails a relatively difficult catalyst removal, since the mineral acid catalyst has to be neutralised and removed with an immiscible liquid entrainer added actually before the esterification (E. Breitmaier and G. Jung, Organische Chemie I&II, Georg Thieme Verlag, 1994, 3rd edition, p. 271f(I); p. 490(II)).
EP 0192035 describes a process for deacidifying fats or oils, in which acidic solid ion exchange resins are used as catalysts, and whose removal from the reaction mixture is followed by removal of the water by-product. However, a high excess of methanol is required in this process to achieve a high conversion of the free fatty acids. For example, for a fatty acid conversion of 95%, a molar methanol to fatty acid ratio of 35:1 is required. For the distillative removal of the water by-product from the reaction product, the methanol used in excess likewise has to be evaporated owing to its lower boiling point, which causes a very high energy consumption.
According to EP 0192035 (Example 1), an addition of 0.2 l of methanol per 1 l of oil with an acid number of 10 (corresponds to a content of free fatty acids of 5% by weight) is required. In order to achieve a conversion of the free fatty acids of 90%, i.e. a reduction in the acid number from 10 to below 1, according to EP 0192035, an amount of catalyst of 7 liters per liter of oil per hour is needed, which gives rise to considerable reactor volumes and correspondingly high capital costs.
DE 19600025 describes a two-stage process for esterifying free fatty acids with heterogeneous catalysts analogously to EP 0192035, wherein the water by-product is removed between the two reaction stages and the amount of alcohol required is divided uniformly between the two reaction stages. When the esterification is used as a preliminary stage for a transesterification reaction with the same alcohol, this enables a reduction in the energy expenditure for the distillative removal of water and excess alcohol, since the alcohol can remain in the reaction mixture after the second reaction stage.
DE 19600025 further discloses that, in the case of the maximum esterification conversion of 90% disclosed, a maximum acid number of no more than 60 mgKOH/g may be present in the starting material (corresponds to a concentration of free fatty acids of approx. 30% by weight); this among other factors results in the calculated, only very low space-time yield of 34 g of fatty acid methyl ester per liter of reactor volume and hour in the disclosure according to DE 19600025. At a relatively low molar ratio of methanol to fatty acid of 17.5:1, only 85% fatty acid conversion is achieved.
A cause which can be cited for these disadvantages in DE 19600025 is the disclosure that operation of the process is possible only up to 70° C. using ion exchange resins, since there is thought to be a risk of inadequate stability of such a catalyst. For operation under temperatures higher than these, the use of silica gel-based catalysts is proposed, but these appear disadvantageous for the following reasons.
Under some circumstances, all catalysts exhibit the phenomenon of leaching, which is known to those skilled in the art and encompasses the discharge of catalyst material into the product. It is advantageous in this context when the catalyst consists of substances which are at least chemically similar to the starting materials, or products of the process within which it is used, in order that contamination of the product by leaching has a lesser adverse effect on the product quality. In connection with the preparation of alkyl esters, the use of hydrocarbon-based catalysts, which also include the ion exchange resins, is thus advantageous.
The person skilled in the art is also aware that increasing the temperature generally increases the reaction rate of a chemical reaction and hence, within a given time, an increased, economically advantageous conversion to product can be achieved. DE 19600025, however, points out the inadequate thermal stability of the ion exchange resin catalysts and the use of other heterogeneous acidic catalysts in applications under high temperatures.
For an economically viable and energetically optimal esterification of free fatty acids in vegetable and animal fats and/or oils with alcohols for biodiesel production, it is thus an object of the invention to develop a process which reduces the content of free fatty acids to the demands of a downstream transesterification stage, by virtue of the reaction achieving a high conversion of the free fatty acids at elevated temperatures and very low alcohol excess, such that the process enables an improvement in the space-time yield and hence in the required apparatus size as compared with the prior art processes.